Two important examples of limitations and exceptions to copyright are the fair use doctrine found in the United States, and the fair dealing doctrine found in many other common law countries. Other more fundamental boundaries of copyright are caused by thresholds of originality (below which objects cease to be copyrightable), the idea-expression dichotomy, the public domain and the effect of Crown copyright. These might be interpreted as defining copyright, rather than being "limitations" or "exceptions" to it.
The scope of copyright limitations and exceptions is currently a subject of significant controversy within various nations, largely due to the impact of digital technology, and the enactment of anti-circumvention rules in response to the WIPO Copyright Treaty. Defenders of copyright exemptions fear that DRM technology will massively reduce the scope of important exceptions. Their opponents believe that if existing exemptions are allowed to continue, they will necessarily allow huge amounts of private copying and/or piracy-- if consumers can make a copy of a CD for their car, they can give MP3s to everyone.
Limitations and exceptions are also the subject of significant regulation by international treaties. These treaties have harmonized the exclusive rights which must be provided by copyright laws, and the Berne three-step test operates to constrain the kinds of copyright exceptions and limitations which individual nations can enact.
On the other hand, international copyright treaties place almost no requirements on national governments to provide exemptions from exclusive rights (one notable exception to this is Article 10(1) of the Berne Convention, which guarantees a limited right to make quotations from copyrighted works).